1. Field of the Invention
The present invention relates to ink jet printing apparatus, e.g. of the continuous type, and more specifically to a structural and functional system that provides improved start-up/shut-down modes for such apparatus.
2. Description of the Prior Art
The term "continuous" has been used in the field of ink jet printer apparatus to characterize the types of ink jet printers that utilize continuous streams of ink droplets, e.g in distinction to the "drop on demand" types. Continuous ink jet printers can be of the binary type (having "catch" and "print" trajectories for droplets of the continuous streams) and of the multi-deflection type (having a plurality of print trajectories for droplets of the continuous streams). Binary type apparatus most often employs a plurality of droplet streams while multi-deflection apparatus most often employs a single droplet stream.
In general, continuous ink jet printing apparatus have an ink cavity to which ink is supplied under pressure so as to issue in a stream(s) from an orifice plate in liquid communication with the cavity. Periodic perturbations are imposed on the liquid stream(s), e.g. vibrations by an electromechanical transducer, to cause the stream(s) to break up into uniformly sized and shaped droplets. A charge plate is located proximate the stream(s) break-off point to impart electrical charge in accord with a print information signal and charged droplets are deflected from their nominal trajectory. In one common (binary) printing mode, charged droplets are deflected into a catcher assembly and non-charged droplets proceed to the print medium.
The components described above (particularly the orifice plate and charge plate) should be precisely sized and positioned to achieve accurate placement of droplets on the print medium or on the catcher face. However, even after such careful manufacture, significant problems often are presented at each operational start-up of ink jet printers. First, any dried ink residue remaining from previous usage presents serious problems. For example, if such residue is on the charge plate it can cause shorting or improper charging of droplets or interfere with the droplet trajectory. If the residue is on the lower print head structure (e.g. the operative catcher surface), it can cause ink splatter. Also, it is quite difficult to initiate the continuous droplet stream(s) along their nominal trajectories, without some initial jet instability causing a partial wetting of the charge plate.
Prior art solutions to avoid charge plate shorting due to ink contamination have included (i) manually cleaning the charge plate; (ii) providing a nearly instantaneous negative pressure at shut-down to avoid creating residue on the lower print head; (iii) moving the lower print head charge plate structure away from its operative position at start-up and (iv) providing a rapid pressure pulse in the image bar to force an initially straight start for the ink jets.
These solutions are all useful, but not without related difficulties or disadvantages. Manual cleaning of the charge plate is not desirable, particularly for office environment applications. Moving of the charge plate to avoid wetting during start-up adds mechanical complexity and causes great potential for unaccuracy in its proper alignment with the upper print head assembly's orifice plate. Using the "water-hammer" approach to achieve instantaneous start-up of the jets in their printing trajectory requires an extremely fast-actuation solenoid valve and rigid conduits. This approach is sometimes unreliable in constructions where jet-to-electrode clearances are very small. Instant shut-down of the jets to avoid ink contamination on the charge plate has similar disadvantages and, in itself, will not solve the problem of accumulated residue on the lower print head structure.